INTRODUCTION
2.3 Stagnation-Plane Flat-Flame Burner
The burner used in the present work is a stagnation-plane-stabilized flat-flame burner.
Here, a laminar jet of premixed fuel/air with a top-hat velocity profile is directed normal to a planar surface. The flame stabilizes between the nozzle exit and the stagnation plane. Depending on the
operating conditions, the flame either attaches itself to the nozzle exit as a Bunsen flame, or establishes itself as a flat flame in free space.
This burner configuration has been used by Vagelopoulos and Egolfopoulos (1998) as well as Bergthorson and Dimotakis (2005) in experiments to determine laminar flame speed. By operating near the transition point between the attached and detached conditions, the flame strain rate is near zero, and laminar flame speeds can be measured.
The burner for the present work was chosen to be similar to that used by Vagelopoulos et al. (1998). The nozzle exit diameter is 14 mm and the height of the stagnation body above the nozzle is 21 mm or 1.5 D. Figure 2-3 shows schematic of the burner assembly as well as a picture of the unit removed from the test section (but still attached to the base plug.) Figure 2-4 shows a sectional view of the nozzle assembly.
The nozzle is constructed from four components forming three distinct chambers. These chambers are separated by two sintered bronze disks, each with a thickness of 2.3 mm and with an average pore size of 30 um. These disks act both as linear restrictors as well as flow distributors.
Premixed methane and air enter the bottom chamber through an integral, precision needle valve and then through a diffuser plug which redirects the flow radially. The differential pressure across the needle valve is kept sufficiently high to maintain choked conditions, thus ensuring there is no feed system coupling in the measured response.
The flow then passes through the first sintered disk and into the middle chamber. The restriction in this disk encourages the flow to distribute itself across it evenly. The flow emerges from the first disk and then passes into the second sintered restrictor. Again the presented pressure drop helps to further balance the flow so that the resulting velocity profile on the upper side of the second disk is uniform. A pressure tap present in the second chamber allows the differential pressure between it and test section (atmosphere) to be measured using a Datametrics Barocel. Due to the small size of the pores in the sintered disk the resulting Reynolds number is very low (10 or
Figure 2-3: Flat-flame burner assembly. (Left) Simplified three-view showing the nozzle, stagnation body and stagnation body support structure. (Right) Photo of burner assembly along with chamber base-plug and interconnecting support rods. Stagnation body height, H, is shown as 21 mm. This corresponds to an H/D = 1.5, which is the value used for all experiments.
Figure 2-4: Flat-flame burner nozzle. Outline view is shown on right while sectional view is shown on left.
less). Consequently, the restriction acts very similar to a laminar flow element. The measured value is used to compute fuel/air flow rate and the corresponding nozzle exit velocity based on the current conditions (equivalence ratio and temperature) and calibration data.
The flow then enters the upper section of the nozzle assembly which contains the aerodynamic contraction. This contraction was designed using the method outlined by Chmielewski (1974) in order to produce an exit velocity that is near top-hat in profile. A contraction area ratio of 10 was chosen along with an L/D of 1.75 resulting in a nozzle with a cavity Lc/Dc on the order of unity.
Each of the three cavity sections in the nozzle assembly contains small ports for 6 mm diameter, integral, cartridge microphones (Panasonic WM-60A). These microphones are used to measure the amplitude and mode shape of the pressure field coupled into the nozzle from the acoustic forcing. Indeed, this data is an essential part of understanding the different contributions resulting in the p’ to q’ coupling.
Once the flow leaves the nozzle it impinges against the planar stagnation body. This body is fabricated in two parts from 304 CRES and is water cooled. The lower portion of the body is black-oxide conversion coated to minimize light scattering during PLIF experiments. Type K thermocouples measure the inlet and exit water temperatures and an internal RdF heat flux patch measures (approximately) the heat flux through the face. The burner heat flux was estimated over its predicted operating range and the thickness of the body material between the stagnation face and the cooling water cavity was chosen to guarantee that the surface temperature would always remain above 100 °C. This, in turn, ensured that condensation would not collect on the face during operation.
The diameter of the planar section of the stagnation body is 5.7 cm, corresponding to just over four times the nozzle exit diameter. For all presented experiments, the distance from the nozzle exit to the stagnation plane is 21 mm which is equal to 1.5 nozzle exit diameters.
The combustible mixture is fed to the burner from the fuel mixing and feed panel. The panel mixes filtered and dried facility air with UHP (ultra-high purity) bottled methane and delivers it to the burner. The methane flow is controlled in an open-loop fashion by a manual flow controller. Its flow rate is measured using a thermal mass flow meter. This measurement, in turn, is used to closed-loop control the flow rate of the air using a thermal mass flow controller. The closed-loop controller operates to target a specific equivalence ratio. The two component streams are passed through a pair of mixers before moving on to the distribution manifold and ultimately to a ported relief valve. This valve regulates the operating pressure of the manifold to 206.7 kPaG, dumping any excess fuel/air mixture overboard to an outside vent. Premixed fuel/air is then tapped from this manifold and directed to the burner. The flow rate to the burner is independently controlled by the needle valve incorporated in its base.
By using the bypass regulation scheme as described (versus direct flow control), the control of the fuel/air equivalence ratio and the flow rate to the burner are effectively decoupled. This allows for far more accurate control of each of these parameters, as well as much improved long term operational stability. Details of the design and construction of both the burner and the fuel feed system are presented in appendix B.